Dogs with muscular dystrophy receive CRISPR treatment: What does this mean for humans?

In an exciting step forward, dogs with Duchenne muscular dystrophy (DMD) were treated with the CRISPR genome editor to restore production of the missing protein responsible for the disease. After more studies to prove it is safe and effective long-term, this has the potential to move into clinical trials as a DMD therapy.

“[The dogs] showed obvious signs of behavioral improvement – running, jumping – it was quite dramatic,” lead researcher Eric Olson, a molecular biology professor at the University of Texas Southwestern Medical Center, told Wired. Although Olson commented on the dogs’ behavior, it’s important to note that there was no video proof nor evidence reported in the paper on the dogs’ symptoms post-treatment.

DMD, characterized by the body’s inability to make a certain protein (dystrophin), is an X-linked genetic disorder that affects about 1 out of every 3,600 boys worldwide. Progressive muscle degeneration and weakening can begin as early as 3 years old and affect every muscle in the body, eventually leading to life-threatening complications from heart and respiratory muscle weakness.

CRISPR was previously used to restore dystrophin expression in mice with DMD. The next step in moving this treatment to the clinic involves testing it in a larger animal model, like dogs. Dogs with DMD offer something mouse models can’t – the dogs display many similar symptoms to humans affected by DMD, such as muscle weakness and wasting.

This study used DMD beagles with a genetic mutation similar to the most common mutation in humans, making it an important model for the clinical translation of CRISPR gene editing. Although this mutation is the most common, only about 13% of boys with DMD have it. If this were to become an approved therapy, it means a maximum of 13% of DMD patients could be helped.

In this study, four 1-month-old beagles were treated with dystrophin-targeted CRISPR either directly in a leg muscle (2 dogs) or intravenously at either a low or high dose (the other 2 dogs). Six weeks after muscle injection, the dogs’ leg muscle was making dystrophin at about 60% of healthy levels, compared to almost none pre-treatment. After 8 weeks, the dog treated intravenously with a high dose made dystrophin at 5-92% of healthy levels in various muscles, notably 92% in the heart and an average of 50% restoration in three leg muscles. A previous study suggested that only 15% of healthy dystrophin expression is needed to ease some DMD symptoms in mice.

The image below shows restored dystrophin levels (green) in the biceps of a healthy dog (1st panel from the left), an untreated DMD dog (2nd), and the dogs treated with a low (3rd) or high (4th) intravenous dose of CRISPR.

“Viruses are promising and effective tools for gene therapy, but there are potential dangers. Using even a harmless virus for gene therapy could result in a life-threatening immune response against the virus,” said Jason Nomburg, a virology graduate student at Harvard University. “This is especially true when the virus is administered systemically to the blood at high concentration, like in this study. A second injection can be especially dangerous because it has even greater potential to provoke an immune response. Without knowing how durable the treatment is long-term, this might be a one-time shot.”

While these results are certainly promising, it is important to be aware of the limitations. The small sample size (4 dogs) and short-term monitoring (6-8 weeks) means we should interpret them with caution. Long-term studies with more dogs are needed for verification before even considering clinical trials.

Safety is another key concern, particularly with “off-target” effects where the genome is cut in an unwanted spot. Luckily, none of the three possible off-target sites examined in this study were affected. Importantly, there was little to no information about the dogs’ symptoms and whether they regained muscle function post-treatment. Olson plans to set up longer-term dog studies to examine the treatment’s safety, slotted for completion in 2019. “We just have to be really, really, really careful with this,” Olson told Wired. “We don’t want to have any slip-ups from trying to move too quickly.”

Should we be enthusiastic about these results or are they too good to be true? Only time (and more testing) will tell.

Acknowledgements: Thank you to Jason Nomburg for his commentary on the limitations and implications of this CRISPR gene editing therapy.